Sensors of this type are used in automobile technology, for example, for measuring the level of engine oil or fuel. A sensor on the container floor emits ultrasound impulses. The echo from the fluid surface is reabsorbed by the transmission receiver. The filling level is proportionate to the sound run time. According to the German patent application DE 33 30 059 A1, the sound is guided through a hollow conduit or sound conducting tube which is arranged in a container. At the lower end of the sound conducting tube, the ultrasound transmission receiver is attached. The tube is positioned in the fluid and is filled with fluid via at least one offset opening until the filling level corresponds to that in the container. In the sound conducting tube, which can be curved, the filling level is measured using ultrasound. An embodiment of this type is primarily designed for measuring the filling level with irregularly formed fluid containers. A great disadvantage of this arrangement is that foam from the fluid to be determined can penetrate into the sound conducting tube and slightly falsify the determination of the fluid level.
The problem of foam formation with a sensor based on ultrasound to determine the fluid level lies, in particular with engine oil, in the fact that due to circulations in the oil while the engine is running, air bubbles of different sizes are created. These air bubbles have the property, depending on their size, of either scattering or reflecting the ultrasound signals. Under these circumstances, a sufficiently precise, error-free measurement cannot be guaranteed.
A previous approach to solve the problem by using a fine-mesh filter (mesh width approx. 60 μm) to keep the air bubbles outside of the measuring chamber, does not give a satisfactory result. The penetration of air bubbles is prevented by the filter, but this method fails as a result of the dirt particles and other impurities in the oil. Due to these particles, the filter becomes clogged after a very short time, so that it is no longer possible to equate the level in the measuring chamber with the level of oil in the engine that is to be determined. Thus, the operation of the sensor in the engine cannot be guaranteed for the entire operational life span.
Against this background, the object of the invention is to provide an ultrasound sensor of the type described in the introduction, which due to its geometric structure prevents air bubbles from penetrating into the measuring chamber, and which thus enables a permanent and reliable determination of the fluid level.